1. Field of the Invention
The present invention relates to the fabrication of high value, high precision integrated circuit resistors using a focused ion beam.
2. Description of the Related Art
Integrated circuit resistors are conventionally formed by diffusion of dopant materials as described in a textbook entitled "Basic Integrated Circuit Engineering", by Hamilton and Howard, McGraw Hill, 1975, pp. 16-21, and by deposition or implantation of lightly doped polysilicon as described in a textbook entitled "Polysilicon for Integrated Circuit Applications", by T. Kamins, Klower Academic Publishers, 1988, pp. 212-215. The former reference teaches how conventional diffused resistors may be formed in a maze geometry to conserve space on the chip.
The drawback of both of these methods of integrated circuit resistor fabrication is low accuracy or precision. The values of resistors fabricated by either method may vary by as much as 20% to 100%. The resistance of polysilicon resistors also varies rapidly in response to changes in temperature. These limitations are intolerable in applications such as current dividing resistors for focal plane arrays which must have extremely precise resistance values at a given operating temperature.
A known technique for adjusting the value of an integrated circuit resistor after it has been formed is to ablate away a section thereof using a laser beam. However, the accuracy of this expedient is limited by the relatively large diameter (on the order of one micrometer or more) of conventional laser beams, and its usefulness is limited in that the value of the resistor can only be increased by ablation.